The present invention relates to adaptive equalizer training procedures for use with digital data communication systems. More particularly, the present invention relates to an efficient adaptive equalizer training procedure that decouples the compensation of digital impairments, such as robbed bit signaling, from the compensation of analog impairments.
Digital communication systems may employ a number of initialization, training, and adaptive learning protocols that are designed to equalize the channel distortions, optimize the data transmission speed, reduce transmission errors, and improve the quality of the received signal. For example, the current generation of pulse code modulation (PCM) modems, i.e., modem systems compliant with ITU-T Recommendation V.90, perform an initial training procedure to adaptively adjust the equalizer structure resident at the client-side analog modem (APCM). In addition, an echo canceler architecture resident at the server-side digital modem (DPCM) may be adaptively trained during an initialization period such that the echo channel associated with the DPCM is adequately emulated.
V.90 modem systems perform an initial two-point training procedure during which one constellation signal point (based on a particular xcexc-law or A-law level) is transmitted as a sequence having positive and negative signs. The DPCM transmits the two-point training sequence to the APCM, and the APCM analyzes the received signal to determine the channel characteristics and to adjust its equalizers. FIG. 1 illustrates a typical V.90 modem downstream transmission channel 100 over which such a training sequence may be sent. The signal b(n) may represent a sequence of digital symbols, e.g., 8-bit codewords, that are to be transmitted by a DPCM transmitter 102, where xe2x80x9cnxe2x80x9d represents the time index for the transmitted symbol.
A number of digital impairments 104, such as robbed bit signaling (RBS) and digital pads, may be present within the digital network channel associated with the DPCM. A digital to analog conversion occurs at a codec 106 to facilitate transmission to the end user over an analog loop. Analog impairments 108, such as nonlinear and linear distortion, may be associated with the analog loop and/or any number of analog processing components. Furthermore, in practical applications, additive noise 110 may be introduced to the analog signal before the analog signal is received by the APCM receiver 112.
In practical V.90 systems, RBS-altered symbols are usually periodic in nature based on the symbol count, e.g., RBS may occur every six or twelve symbols. Furthermore, the effect of RBS is deterministic but unknown to the APCM, while digital pads cause a constant, deterministic, and level-dependent (nonlinear) effect. Prior art modem systems may compensate for linear analog impairments, such as amplitude and phase distortions, with well known linear equalization techniques. However, such techniques may not adequately compensate for the presence of digital impairments, which may alter the level associated with the predetermined training point. Consequently, the initial training procedure performed by conventional V.90 modem systems may not provide the most efficient and effective result.
Digital impairments may also cause similar problems during the training of the DPCM echo canceler, which may be configured as an adaptive equalizer structure. The presence and effects of downstream (and upstream) digital impairments are not known by the DPCM and conventional echo canceler training techniques may not adequately compensate for digital impairments located within the DPCM echo channel. Thus, such conventional echo canceler training procedures may not provide an optimum and efficient solution.
Accordingly, an advantage of the present invention is that it provides an improved technique for training an equalizer structure of a digital data communication system in response to a two-point training signal.
Another advantage of the present invention is that it provides an equalizer training technique that compensates for the presence of digital impairments such as RBS.
A further advantage is that the two-point training procedure decouples the compensation of digital impairments from the compensation of analog impairments.
Another advantage of the two-point training procedure is that it may be utilized during the training of echo canceler structures.
The above and other advantages of the present invention may be carried out in one form by a method of training an equalizer structure in a data communication system having a communication channel susceptible to deterministic, periodic digital impairments and analog impairments. Such a method may include the steps of: transmitting, over the communication channel, a sequence of digital codewords having positive and negative signs, where each of the digital codewords represents the same magnitude; obtaining an impaired digital sequence generated in response to the sequence of digital codewords, where the impaired digital sequence may be affected by the digital and analog impairments; compensating for the effect of the digital impairment on the impaired digital sequence, where the compensating step is decoupled from the compensation of the analog impairments; and adjusting an adaptive equalizer element in response to the compensating step, the adaptive equalizer element being employed to equalize the impaired digital sequence.